1. Field of the Invention
The present invention relates to an improved catalyst for the hydration of olefins with steam to give alcohol products.
2. Description of the Background
It is known that olefins can be reacted with steam in the gas phase at elevated pressures to give alcohols. Such processes have become particularly important industrially for the production of ethanol from ethylene and isopropyl alcohol from propylene. The synthesis of these alcohols is carried out in the presence of a catalyst, the catalyst generally used being phosphoric acid, which is applied to carriers. Known carriers are either those based on pure silica, such as, for example, kieselguhr or silica gel (U.S. Pat. No. 2,579,601) or those based on silica with a higher or lower alumina content, such as, for example, calcined diatomaceous earth, the structure of which is held together by clay or clay-containing substances (German Patent 2,722, 616 and U.S. Pat. No. 3,311,568).
In the case of the carriers on pure silica, the strength over a prolonged time-on-stream presents problems. Although the alumina-containing materials are distinguished by better mechanical strength, when they have an excessively high alumina content, they have the disadvantage that the alumina is dissolved away during the reaction by the action of the phosphoric acid.
German patent 1,156,772 describes a process for the preparation of an alumina-containing carrier for the phosphoric acid used as a catalyst in the hydration of olefins, in which process catalyst moldings of mineral aluminum silicates are treated with a mineral acid in such a way that the alumina content is preferably reduced to between 1 and 5 per cent by weight. This material generally has both the required mechanical strength and a sufficiently low residual aluminum content to avoid removal by dissolution. On the other hand, when commercial catalyst elements are used for the preparation of catalyst carriers for the hydration of olefins, it has been observed that, without preselection of the raw material, very different catalyst activities are found.
Finally, it has been possible to develop, also based on coarse-pore silica gels, carriers for phosphoric acid having high hydration activity and sufficient mechanical strength (German Patent 2,525,705 and 2,719,055).
However, the remaining disadvantage of these carriers based on amorphous silica is that the amorphous silica partially crystallizes during prolonged use under conditions of the hydration reaction, which is associated with a sharp reduction in the specific surface area and hence in the catalytic activity, in an irreversible manner, and with a decrease in the mechanical strength.
A further disadvantage of all hydration catalysts used to date, which are based on phosphoric acid on a silicate carrier, is the slow decrease exhibited in their activity as a result of the discharge of phosphoric acid, which, during continuous operation, must be constantly neutralized with an alkali in order to avoid corrosion effects of the acidic crude alcohols on downstream apparatuses. By continuously spraying in phosphoric acid according to German Patent 2,658,946 in an amount which corresponds to the amount of phosphoric acid discharged, it has been possible substantially to avoid the continuous loss of activity and hence considerably prolonging catalyst life; however, this involves setting corresponding requirements for the life of the carrier, so that it is not possible to use such carriers in which, under reaction conditions, crystallization occurs with a reduction in the catalytic activity in an irreversible manner and the mechanical strength decreases over the course of time.
As indicated in German Patent 2,908,491, a carrier for a hydration catalyst of constantly high catalytic activity can be obtained from clay minerals when, by a careful choice of the raw material, care is taken to ensure that the material consists to a large extent of montmorillonite, which results in a large surface area and a large absorption volume after shaping, leaching and impregnation. The resulting catalysts or catalyst carriers of montmorillonite-containing clay have a higher activity than those prepared from molded catalysts based on mineral aluminum silicates of different origin, i.e. about 105 to 110 g of ethanol or about 300 g of isopropyl alcohol are obtained per hour per liter of catalyst bed. However, this increased activity can only be maintained over a relatively long period, if the discharged phosphoric acid, which amounts to about 0.07 g per hour and per liter of catalyst bed in the case of ethanol and about 0.01 g per hour and per liter of catalyst bed in the case of isopropyl alcohol, is compensated by the continuous addition of the same amount of acid. This discharged acid must moreover be neutralized with an alkali. The mechanical strength of the catalysts is of the order of magnitude of 70 to 90 Newton/sphere, which is sufficient for the loading of the conventional reactors. Thus, once a catalyst having an adequate life and long-term strength has been found and the discharge of phosphoric acid has been suppressed to a satisfactory value, the carriers consisting predominantly of silica gel remained superior to the carriers prepared from montmorillonite only with regard to their initial activity. Thus, according to German Patent 2,722,616, up to 144 g of ethanol per hour and per liter of catalyst bed are produced, but only 115 g of isopropyl alcohol per hour and per liter of catalyst bed. U.S. Pat. No. 3,311,568 describes a substantially higher catalyst activity, namely 240 g of ethanol per hour and per liter of catalyst bed, phosphoric acid being metered in every 24 hours and in general the activity being observed only over 1500 hours, i.e. about two months. It is reported that the strength increases with a higher content of bentonite, and the mixing of 3 to 5% of bentonite with the diatomaceous earth prior to sintering is described.
Finally, according to German Offenlegungsschrift 3,709,401, it was found that the good catalyst properties according to German Patent 2,908,491 and German Patent 2,929,919 with regard to long-term strength and good retention of the phosphoric acid are maintained in terms of the amount of alcohol produced per unit time and catalyst volume, can be considerably increased, this activity remaining virtually unchanged over a period of six months, if 20 to 40 per cent by weight, based on the total dry substance, of a fine-particled silica gel are added to the clay treated with acid in the first stage and having a high montmorillonite content, prior to molding for calcination, and the surface area and pore volume of the finished catalyst carrier is thus increased. Since the montmorillonite carrier alone in the leached state prior to impregnation with phosphoric acid has a specific surface area of only 150 to 160 m.sup.2 /g and a pore volume of about 0.7 ml/g, it is not difficult to see that, as a result of the addition of, for example, 30% of silica gel (specific surface area about 350 m.sup.2 /g), the specific surface area increases only linearly to 180-200 m.sup.2 /g, whereas the pore volume (addition: silica gel 1.0 to 1.2 ml/g) increases disproportionately to 0.95-1.0 ml/g, which is critical for the increase in activity. The pore diameters of the finished carrier prior to impregnation with phosphoric acid are between 1 and 20.multidot.10.sup.-9 m and the maximum of the frequency distribution is about 5.multidot.10.sup.-9 m. In the process according to German Patent 3,709,401, it has been found that an increase in the compressive strength of the spheres, which occurs in the case of the natural product montmorillonite on subsequent calcination, is transferred to the mixed carrier even under process conditions, whereas pure silica gel always suffers a loss of strength under process conditions. A need therefore continues to exist for an olefin hydration catalyst carrier of improved strength.